The present invention relates to an apparatus for examining a surface of a substrate in a lighting environment similiar to that of outside daylight. A specific embodiment of the invention is implemented in an apparatus for examining a thin layer of metallic oxide such as an SnO.sub.2 based substance deposited on a glass plate or strip. The thickness and uniformity of the layer is very important since variations in thickness tend to quickly induce variations in tint and other undesirable effects.
Evaluation of the quality of these layers is performed partly by visual examination of the layers, especially in the case of non-uniform layer thickness which gives rise to undesirable tint characteristics visible to an observer in daylight.
The most accurate and reliable observations of these layers are made in outside daylight. Observations in the workshop of these layers inevitably include reflection from surrounding superstructures which tend to place the glass in a false light and thereby make proper examination difficult. However, it is desirable to examine the coated glass at the place of production, before being brought to outside daylight, so that possible production problems may be immediately remedied in case of unsatisfactory quality. In addition, such an immediate observation is advantageous in that there exists no dependence on the daily or seasonal variations of illumination of the sky.